This invention relates to a process for use in a drying system wherein combustion gases are recycled through the drying system prior to being vented to the atmosphere.
Drying systems are important features in the manufacture and processing of many different materials. For example, drying systems are often used to dry wood chips during the manufacturing of particle board. Further, drying systems are of particular importance during the processing of ethanol. More particularly, after ethanol has been removed from grain during the fermentation process, it is then desirable to dry the grain to allow storage and resale of the grain for animal feed or other uses.
Typical drying systems include a combustion chamber into which natural gas and air are supplied and combusted. The heated combustion gases in the combustion chamber are then induced by a draft fan into a rotating cylindrical dryer. The material to be dried is introduced into the dryer and exposed to the current of heated gases. The dried material is then separated from the heated gas current in a cyclone separator. The remaining heated gases are then vented to the environment. An example of a typical drying system of the prior art is disclosed in U.S. Pat. No. 3,861,055, which is incorporated herein by reference.
Numerous problems and disadvantages are associated with these prior art drying systems. A major problem involves the venting of the combustion gases to the atmosphere. More particularly, these combustion gases contain various pollutants. For example, the gases oftentimes contain volatile organic compounds (VOC's), carbon dioxide (CO.sub.2), and nitrous oxide (N.sub.2 O). In addition to pollutants that result from the combustion process in the combustion chamber, pollutants can also result from the drying of the material itself. For example, when drying grain used to produce ethanol, a small percentage of the ethanol remains in the grain and is evaporated during the drying process. Thus, this evaporated ethanol becomes part of the heated stream of gases exiting the dryer and entering the atmosphere. Because of governmental standards that set the level of pollutants that can be vented to the atmosphere, it is often necessary to add additional pollution control devices to the drying systems to reduce the pollutant levels in the gas stream prior to venting. These devices often are add-on oxidizers which oxidize the VOC's present in the gas stream to reduce the VOC's to an acceptable level. These pollution control devices are typically expensive to install and operate.
Another disadvantage associated with prior art drying systems and processes involves the fire hazard associated with excessive amounts of oxygen (O.sub.2) in the combustion gases. More particularly, in order to convey the material to be dried through the dryer, a large volume of moving gas is needed. This is especially true when the material contains a large percentage of moisture. Typically, drying systems make up the necessary volume by introducing excess air during the combustion process in the combustion chamber. Although this results in a suitable volume gases to convey the materials, it also results in an excessive amount of O.sub.2 in the combustion gases. In many instances, the amount of O.sub.2 exceeds the allowable fire and explosion standards. The use of large amounts of excess air also results in other problems with these drying systems. More particularly, increasing the excess air admitted to the combustion chamber results in a decrease in the temperature of the combustion gases exiting the burner.
In order to reduce the amount of O.sub.2 in the combustion gases and increase the temperature level of the combustion gases to a suitable level for drying, attempts have been made to decrease the amount of excess air introduced into the combustion chamber. However, reducing the amount of excess air results in various other inherent disadvantages with the drying system. More particularly, as is apparent, decreasing excess air results in a lower volume of gas flowing through the drying chamber. This can result in ineffective and/or unstable pneumatic conveying of the product through the drying system. Further, oftentimes the excess air is decreased to raise the temperature of the exiting combustion gases above the normal drying level in order to compensate for the decreased volume of gases moving through the drying chamber. This elevated temperature, however, can result in thermal degradation of the material being dried. Further, as the temperature of the combustion gases is raised higher and higher, the amount of the pollutant N.sub.2 O and the amount of VOC's dramatically increases. N.sub.2 O is an especially undesirable and difficult to treat pollutant.
Some prior art drying processes use more than one dryer to ensure that the material is adequately dried. In these processes, the moisture content of the material is reduced in a first dryer to a particular level using a medium range temperature, and then completely dried in additional dryers also using medium range temperatures. This type of drying process, although producing adequate drying results at medium range temperatures, may still produce large amounts of pollutants. In other prior art drying systems having a single combustion chamber and a single dryer, attempts have been made to recycle the entire quantity of combustion gases exiting the dryer back to the combustion chamber. However, because the entire quantity of combustion gases was recycled, these systems often had to operate within very narrow operating parameters. If the system was ever operated outside these narrow parameters, the efficiency and operation of the system were adversely affected. For example, the combustion process could be interfered with if the system strayed outside the parameters.
Therefore, a drying process is needed that oxidizes pollutants within the system so that external pollution control devices are not needed. Further, a drying process is needed which decreases the amount of O.sub.2 present in the system to a level below fire standards without affecting the efficiency of the dryer due to the lack of available conveying gases. Still furthermore, a drying process is needed which will maintain a suitable temperature that limits the production of N.sub.2 O as a pollutant and limits thermal degradation of the dried material.